Researchers have managed to answer a deceivingly simple question of how baby salmon were able to swim out of their nests in the gravel to reach the surface.
When salmon spawn, their fertilised eggs are buried in gravel nests where they remain until they hatch. When newly hatched, the baby salmon first feed on the residual yolk stores. Once this is depleted, they will make their way to the surface and live in the open waters.
But, while they are buried in the gravel, in the absence of signposts and a periscope, how do these baby salmon know which way is up?
According to a study published recently Iin the journal Biology Letters, it seems that they use the earth’s gravitational field to orient themselves.
"Getting out of the gravel is not as easy as it might seem, but it is of critical importance," said senior author David Noakes, director of the Oregon Hatchery Research Center at Oregon State University. In their study, he and his team discovered that the fish uses magnetic cues for three-dimensional orientation across a wide range of spatial scales and habitats.
“This matters because we need to know how rearing conditions might impact the fish, particularly in the case of hatcheries—where we already have some evidence that exposure to unnatural magnetic fields can disrupt the ability of steelhead trout to orient appropriately.”
In previous studies, he had examined the role of temperature, light, and water current on salmon emergence. For this particular study, the team constructed a system of copper-wire coils and ran a very low electric current through the wire coils to control the magnetic field surrounding the fish.
Two groups of fish that were developmentally ready to move into the surface waters were placed at the bottom of plastic tubes filled with clear glass marbles (to mimic gravel). One group was exposed to a normal magnetic field while the other was exposed to an inverted magnetic field.
The distance the fish swam up the tubes within a 30-minute period was measured.
According to co-lead author Nathan Putman, senior scientist at LGL Ecological Research Associates in Bryan, Texas, “Given that only inverting the magnetic field influenced fish movement, it seems salmon use the direction of field lines to orient vertically during their emergence from gravel—our findings are difficult to interpret in any other way.”